Question

Consider the following table in SQL Server: Tasks (Payload nvarchar, DateToExecute datetime, DateExecuted datetime null).

Now we have two worker processes (2 Azure worker role instances in our case). Both of them periodically try to get records where DateExecuted IS NULL AND DateToExecute <= GETDATE(). Then they process that record and set (SQL update) DateExecuted to current date.

The problem is that a single task should be processed only once by a single worker instance.

What’s the best way to provide synchronization or locking for implementing such scenario?

Answer 1

The easiest way to do locking over multiple roles/instances in Windows Azure is by using blob leases. Steve Marx created a great class for this called AutoRenewLease (source, NuGet, blog post). If you already have a timer or while loop, you can write code like this:

using (var arl = new AutoRenewLease(leaseBlob))
{
    if (arl.HasLease)
    {
        // Query Tasks table and do work....
    }
    else 
    {
        // Other worker is busy....
    }
}

Or you could use the DoEvery method which allows you to schedule your code every X minutes:

AutoRenewLease.DoEvery(leaseBlob, TimeSpan.FromMinutes(15), () => {
    // Query Tasks table and do work....
});